Weisskopf and colleagues tracked pesticide breakdown products in the children's urine and found those with high levels were almost twice as likely to develop ADHD as those with undetectable levels.

The findings are based on data from the general US population, meaning that exposure to the pesticides could be harmful even at levels commonly found in children's environment.

"There is growing concern that these pesticides may be related to ADHD," says Weisskopf.

"What this paper specifically highlights is that this may be true even at low concentrations."

Nerve toxins

Organophosphates are known to be toxic to the nervous system and were used as chemical warfare agents.

There are about 40 organophosphate pesticides registered in the United States. Organophosphates are also registered for use in Australia.

Weisskopf says the compounds have been linked to behavioural symptoms common to ADHD - for instance, impulsivity and attention problems - but exactly how is not fully understood.

Although the researchers had no way to determine the source of the breakdown products they found, Weisskopf says the most likely culprits are pesticides and insecticides used on produce and indoors.

Garry Hamlin of Dow AgroSciences, which manufactures an organophosphate known as chlorpyrifos, say he has not had time to read the report closely.

But, he adds "the results reported in the paper don't establish any association specific to our product chlorpyrifos."

Weisskopf and colleagues' sample included 1,139 children between 8 and 15 years. They interviewed the children's mothers, or another caretaker, and found that about one in 10 met the criteria for ADHD, which fits with estimates for the general population.

After accounting for factors such as gender, age and race, they found the odds of having ADHD rose with the level of pesticide breakdown products.

For a 10-fold increase in one class of those compounds, the odds of ADHD increased by more than half. And for the most common breakdown product, called dimethyl triophosphate, the odds of ADHD almost doubled in kids with above-average levels compared to those without detectable levels.

"That's a very strong association that, if true, is of very serious concern," says Weisskopf. "These are widely used pesticides."

He emphasises that more studies are needed, especially following exposure levels over time, before contemplating a ban on the pesticides. Still, he urges parents to be aware of what insecticides they were using around the house and to wash produce.

"A good washing of fruits and vegetables before one eats them would definitely help a lot," says Weisskopf.

One of many risk factors

Australian ADHD expert, child psychiatrist Professor Alasdair Vance at the The Royal Children's Hospital Melbourne says the US study is "well conducted" but urges caution in interpreting the findings.

He says ADHD is a complex condition which has many risk factors including poverty, adverse family conditions and learning difficulties.

Vance says environmental toxins are likely to be responsible for a very small proportion of ADHD cases.

He says it is likely that most of children in the US study that had high levels of organophosphates were also exposed to other factors that have been linked more strongly to ADHD.

In this case organophosphates may just be a marker for ADHD rather than a causative agent.

"We know around the world that those people exposed to more of these environmental toxins are often also suffering from poverty," says Vance.

"And we know that all biological and psychosocial factors tend to be worse in the context of poverty."

Concerns about chemical exposure

Meanwhile, Jo Immig of Australia's National Toxics Network says the findings are concerning and of relevance to Australia.

"Organophosphates (OP), including chlorpyrifos, are widely used in Australia to produce food and for pest treatments around homes," she says.

Immig says children are at greatest risk from exposure to OP pesticides because their developing biological systems are more at risk of damage during rapid growth.

"And their behavioural patterns make it more likely they will contact contaminated surfaces. For instance, playing on floor surfaces where residues of pesticides are known to accumulate," she says.

Chlorpyrifos is currently under review by the pesticide regulator because "concerns over its toxicity and the potential risks to worker health and safety and the environment".